Centrifugally operated friction clutches are well known in the art of vehicular drive train systems. They typically include an input member driven by a prime mover, usually an electric motor or internal combustion engine, and weights rotatable with the input member which, upon rotation of the driving member, will move radially outwardly under the effect of centrifugal force to cause the input member to frictionally engage a driven output member. Automatically actuated centrifugal clutches employed with heavy-duty electromechanical highway line-haul truck transmissions include so-called centrifugal actuation modules that house the centrifugally actuated weights. The centrifugal modules are drivingly connected to an engine flywheel, and each of a plurality of centrifugally actuated weights is adapted to swing in an arc about a pivot link fixed to the module housing structure. As such, the so-called swing weights contained within the modules are radially outwardly movable against resistive spring forces as a function of engine speed-the higher the speed, the greater the outward movement between limits. Rollers attached to the weights are adapted to roll atop ramp segments that are cammed for clutch engagement and disengagement.
The swing weights are subjected to a number of forces, and thus give rise to competing concerns to achieve satisfactory operation of the modules over the useful life of a clutch. As an example, one feature of the above-described prior art centrifugal clutch is the use of two distinct frusto-conical ramp surfaces on the ramp segments. A first ramp surface exhibits a relatively steep slope and a second ramp surface exhibits a more gradual slope. These ramp surfaces are engaged by swing weight rollers and are used to create a clamp load as the centrifugal force acting on each swing weight increases. Particularly, as the centrifugal force increases, the swing weights will move from their original position on the relatively steep first ramp surface onto the more gradual sloping second ramp surface. Since a centrifugal clutch operates as a balance of forces, any tolerance in the centrifugal module components (e.g., swing weight springs, ramp segments, etc.) may cause a “staggered disengagement”, wherein one or more of the swing weights moves from the second ramp surface to the first ramp surface before the other swing weights. This condition is exacerbated in a swing weight style centrifugal clutch since operation of each individual swing weight is essentially independent of the other swing weights.
Generally, the multi-ramp surface clutch described herein is used with a heavy duty truck (typically greater than about 30,000 lb GVW, more than about 10 liters engine displacement, and having an engine that develops maximum torque typically below about 1200 rpm). The multi ramp surface clutch, as disclosed in commonly owned U.S. Pat. No. 6,880,687, the disclosure of which is hereby incorporated by reference in its entirety, permits an engine that develops maximum torque at a lower rpm to generate a sufficient clutch clamping force to transmit the maximum torque.
Another feature of the above-described prior art centrifugal clutch is the use of mechanical stops for the swing weights. As the swing weights move along the more gradual sloping second ramp surface during engine speed increase, it is desirable to provide a motion limiter for the swing weight to limit its radial travel relative the clutch. The stops prevent the full compression of the springs that are provided to return the swing weights to position during engine deceleration. The stops also provide for a maximum clutch engagement force, as the stops limit the axial displacement caused by the swing weight travel during engine speed increase.
When these stops are used in a medium duty truck (typically about 16,000 to about 30,000 lb GVW, and having an engine that develops maximum torque typically above about 1200 rpm), in conjunction with the more gradual sloping second ramp surface, the swing weights tend to create an undesirable impact noise during engine speed increase that may be audible within the cabin of the vehicle. Additionally, second stops are typically provided to arrest the movement of the swing weights during engine deceleration. Since the conventional multi-ramp surface clutch will disengage as the swing weights traverse the more gradual sloping second ramp surface then the relatively steep first ramp surface, the swing weights may accelerate on the relatively steep first ramp surface and impact the second stops if engine speed decreases dramatically. This undesirable impact of the swing weights and the second stops will typically create an undesirable impact noise.
Accordingly, a need exists for an improved centrifugal clutch for a medium duty engine that avoids staggered disengagement of the centrifugally operated weights and reduces the undesirable impact noise associated with the stops.